The present invention relates to a method for the preparation of a relief image, more specifically for the preparation of a photoresist image, a lithographic printing plate precursor or a flexographic printing plate precursor.
Relief images such as resist images or printing plates are generally produced using positive-working or negative-working UV-sensitive materials that are exposed to UV-radiation. The exposed (positive-working) or unexposed (negative-working) areas of UV-sensitive layer are then removed, generally by wash-out with a developing solution, to form the relief image. During the UV exposure, conventionally, a film carrying a negative or positive image is used as a contact mask. In computer-to-plate or computer-to-film technology, the conventional contact mask can be replaced by a mask which is formed by image-wise exposing a recording layer which is integrated within the material. Such an integrated mask can be provided by printing with an ink jet printer on an ink receiving layer or by heat-mode exposure of an ablatable imaging layer which is substantially opaque to UV radiation.
U.S. Pat. No. 4,555,471 describes a multilayer image recording material that is formed by applying a resist film comprising an intermediate layer, a mask-forming heat-sensitive layer and optionally a top layer onto the relief forming layer of a recording material.
In WO-A-94 03838 and WO-A-94 03839 a process for making flexographic printing plates is disclosed comprising the construction of two separate elements that are laminated to form a printing plate. The first element is a conventional photopolymerizable element, the second element comprises an IR-sensitive layer and a removable coversheet. A barrier layer can be present either on the photopolymerizable layer or on the IR-sensitive layer. The elements are laminated such that the IR-sensitive layer is interposed between the photopolymerizable layer and the cover sheet or that the barrier layer is interposed between the photopolymerizable layer and the IR-sensitive layer. The barrier layer can include polyamides, polyvinyl alcohols, hydroxyalkyl celluloses, vinyl acetate copolymers or polyesters and is present as an oxygen barrier layer and to avoid monomer diffusion from the photopolymerizable layer into the IR-sensitive layer. In EP-A-741 330 no barrier layer is present and the IR-sensitive layer is laminated directly onto the photopolymerizable layer. The coversheet can be subbed with release layers and has to be removed before image-wise exposure.
WO-A-97 00777 discloses a printing plate that is formed by lamination of two elements A and B. Element A is a support with a photopolymerizable layer. Element B comprises a support that is coated on one side with a laser ablatable layer and on the other side with an adhesive layer. Element B is laminated with the adhesive bearing side onto element A so that the laser ablatable layer becomes the top layer and the support of B is located between the ablatable layer and the photopolymerizable layer. UV-exposure of the photopolymerizable layer through the mask formed in the IR-sensitive layer results in unsharp images due to the thick support which is present between the IR-sensitive layer and the photopolymerizable layer.
In EP-A-767 408 a flexographic printing plate is described that is prepared by laminating an IR-sensitive layer covered by a peelable protective film onto a photopolymerizable element comprising a support, optionally an adhesive layer, a photopolymerizable layer, an interlayer and optionally a release layer. Before image-wise exposure the protective film and optionally the release layer are removed.
Contrary to the material of WO-A-97 00777, no support is present between the photopolymerizable layer and the IR-sensitive layer. However, the lamination step of the material disclosed in EP-A-767 408 is performed by the plate manufacturer, so there is a large time lapse between the lamination and the actual making of the relief image by the end-user. After the lamination, the relief imaging material is packed, shipped to the traders (warehouses), stored and finally transported to the end-user. This process of transporting the relief imaging material from the manufacturer to the end-user takes at least three to four months. Furthermore, generally there is also a (varying) time lapse between the arrival of the relief imaging material at the end-user and the actual use of the material. During this entire period, monomer diffusion from the UV-sensitive photopolymerizable layer into the image recording layer can occur. To slow down this diffusion process EP-A-767 408 provides the solution of introducing an interlayer between the image recording layer and the photopolymerizable layer. However, since this interlayer is optimised to prevent monomer diffusion it does not adhere optimally to the image recording layer. Moreover, the known adhesion layers have poor barrier properties, so it is difficult to find a suitable compromise between good barrier functionality and sufficient adhesion.
In addition, the integrated materials (for use in computer-to-plate or computer-to-resist technology) are still significantly more expensive than the conventional materials for contact exposure.
It is a first object of the present invention to provide an inexpensive method for the preparation of a relief image by direct exposure of an integrated mask.
It is a further object of the present invention to provide such a method wherein the adhesion between the image recording layer and the UV-sensitive material is improved and wherein the extent of monomer diffusion from the UV-sensitive layer to the image recording layer is reduced.
The above mentioned objects are realised by a method having the specific features defined in claim 1.
The diffusion of monomers is avoided because the lamination is carried out by the end-user, just before the preparation of the relief image, thereby reducing the time period during which the UV-sensitive layer and the image recording layer are in mutual contact. As a consequence the interlayer does not need to have good barrier properties and can be optimised to adhere optimally to the image recording layer. Additionally, the present invention enables the use of conventional UV-sensitive materials, commonly applied for contact-exposure, which are less expensive then the imaging materials available for computer-to-plate or computer-to-resist technology.
Another advantage of the present invention is the increased resolution of the obtained relief image because of the absence of a thick layer (support) between the image recording layer (2) and the UV-sensitive layer (6).
Further advantages and embodiments of the present invention will become apparent from the following description and drawing.